This invention relates to improved satellite positioning methods and in particular to improved methods of determining the distance between a satellite and receiver. Satellite positioning systems, such as GNSS, are used to determine accurately the position of a receiver on the earth. This requires an accurate knowledge of the distance between a satellite and the receiver. In order to calculate this distance, the exact time taken for a signal to travel from the satellite to the receiver is required. This is achieved by including in the signal an indication of the exact time that the signal left the satellite. Then, by determining the exact time the signal arrived at the receiver, the time the signal took to travel from the satellite to the receiver can be calculated.
For the receiver to determine the time of arrival of the signal, it must first capture the signal. For this purpose, the signal may contain a section of code known as ranging code. The ranging code is known by the receiver. The receiver performs processing operations on the signal in an attempt to identify the ranging code. Once the ranging code has been identified, the receiver “locks on” to the signal and reads the other information contained in the signal. Locking on to the signal for the first time is known as “acquisition”. Once acquisition has been achieved, it is desirable for the receiver to stay in contact with the satellite. Due to relative movement between the satellite and the receiver, the separation of satellite and receiver is constantly changing and needs to be determined. The process of remaining locked on to the signal is known as tracking.
In known systems, the low signal-to-noise ratio at the antenna of the receiver means that in order to identify the ranging code it is normally necessary to perform a correlation operation on the signal. This involves repeatedly measuring the amplitude of the signal over a period of time, normally at constant time intervals. The measured amplitude at a particular time is multiplied by the amplitude of the known ranging code at the corresponding time. The results of these multiplication operations are then summed. The process is then repeated numerous times. Each time the process is repeated, the sampling operation is delayed slightly relative to the previous sampling operation. When the amplitude summation noted above becomes very large, a good match between the incoming signal and the correlation reference data has been found. The time delay applied by the correlator is then measured. In this way a coarse estimate of the time of arrival of the signal at the receiver can be obtained. However, it is desired to provide a more accurate determination of the arrival time, so that the separation of the receiver and satellite can be more accurately determined. This is particularly important during the tracking stage.
One type of signal transmitted by GNSS satellites is known as Binary Phase Shift Keying (BPSK). In BPSK signals the ranging code information is modulated onto the signal as a sequence of square pulses (chips). The present invention is intended for the reception of another type of signal transmitted by GNSS satellites which is known as Binary Offset Carrier (BOC). In BOC signals a BPSK signal containing the ranging code information is first multiplied by a sub-carrier waveform of higher frequency than the BPSK chip rate before being modulated onto the signal. The subcarrier waveform is a square wave whose frequency is the sub-carrier frequency.
One known way of locking on to a BOC satellite signal is by inputting the received BOC signal into a correlator and using the BOC signal itself (which is known by the receiver) as the correlation reference signal. Using this approach, the variation of correlator output with time has the form of a series of narrow peaks of progressively increasing height until a maximum peak is reached, followed by a series of narrow peaks of progressively decreasing height. This method has the disadvantage that it is costly to perform sampling operations with a sufficiently small time-step that the maximum peak is reliably identified.
An alternative way of locking on to the signal is to only perform correlation on the components of the BOC signal which lie in one or other of the two frequency bands in which its energy is concentrated, known as “sidebands”. The centre of the upper sideband is displaced in frequency from the centre of the entire BOC signal by an amount equal to the frequency of the original sub-carrier signal. The centre of the lower sideband is displaced in frequency from the centre from of the entire BOC signal by an amount equal to the negative of the frequency of the original sub-carrier signal. Extraction of only the upper sideband of the BOC signal can be achieved with a Digital Down Converter (DDC). This results in a signal for input to the correlator that is similar to a BPSK signal and the BPSK form of the ranging code is used as the correlator reference signal. Using this method, which is called a “single sideband” method, the correlator output has the same form as it does for a BPSK signal; it increases linearly to a peak and then decreases linearly. The time of the exact peak of the correlation output is harder to estimate accurately using this method. As a result, the estimate of signal arrival time obtainable using this method is fairly rough.
Other known methods involve modifying the incoming signal and processing the resulting modified signal in an attempt to extract information about the time delay between the incoming signal and the modifying signal, and thereby obtain an improved arrival time estimate. Such methods are complicated and require relatively large processing power and so are expensive.